Study blames plasma flow for quiet sun

Our sun is maintained by converting gravitational energy into electromagnetic energy (e.g., visible and invisible radiations). The temperature is so high that the solar matter exists only in gas of light ions and electrons. These are charged particles. Their motions may take the form of the so-called 'plasma', which has these charges moving in a coherent and collective way, appearing as waves. The dynamics are quite nonlinear and intricate. Associated with such plasma, the electromagnetic fields will also change, and have been well known following a very regular pattern. One way to notice this pattern is by counting the intensity of the so-called sunspots, which are caused by pop-ups of strong magnetic energy flow from interior to the surface, like a cork decorated with flares carrying a huge amount of energy that may partly reach the earth. The latest solar cycle peaked in 2001 and was supposed to end in 2008. However, there is something unusual with this cycle: the sun appears too sluggish, with an extra number of spotless days. Now researchers came up with an explanation, which seems under debate [http://www.wired.com/wiredscience/2011/03/spotless-sun-model/]:

Now, Dibyendu Nandy of the Indian Institute of Science Education and Research and colleagues offer an explanation: A “conveyor belt” of plasma inside the sun ran quickly at first and then slowed down.

Nandy and colleagues at Montana State University and the Harvard-Smithsonian Center for Astrophysics ran a computer simulation of magnetic flow inside the sun for 210 sunspot cycles. They randomly varied the speed of plasma flow around a loop called the meridional circulation, which carries magnetic fields from the sun’s interior to its surface and from the equator to the poles.

Observations suggest that the fastest flow runs around 22 meters per second (49 miles per hour). Nandy’s model looked at speeds between 15 and 30 meters per second (33 to 67 miles per hour).

The model found that a fast flow followed by a slow flow reproduced both the weak magnetic field and the dearth of sunspots observed in the last solar minimum.

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Unfortunately, observations of the sun’s surface seem to directly contradict the new model.

“We’re in this quandary, this clash between theory and observations,” said NASA astronomer David Hathaway, who analyzed 13 years of data from the Solar and Heliospheric Observatory (SOHO) that tracked the movement of charged material near the surface of the sun.

Hathaway agrees that a fast flow can cause weak magnetic fields and fewer sunspots. But his observations, published March 12, 2010 in Science, suggest that the meridional flow was slow in the first half of the last solar cycle, from about 1996 to 2000. Only after the solar maximum did the flow speed up.

“That’s where there’s a problem,” Hathaway said. “We see one thing, they want the opposite to explain the observations.”

Nandy and colleagues point out that the SOHO observations only see plasma moving at the surface of the sun, not in the deep interior where sunspots are born. The surface flows might not reflect what’s going on underneath, he says.

“In an analogy that you might be able to relate to, one could ask, do ripples on the surface of the sea indicate how ocean currents determine the migration of aquatic animals deeper inside?” Nandy said.

Hathaway argues that changes in the surface should be transmitted to the interior at the speed of sound, and should reach the creation zone in half an hour or less. The disagreement between theory and data means there must be a problem with the models, he says.